1. Field of the Invention
The present invention concerns a process for removing all ambiguity from measurements by the Doppler effect of the speed of a target. The term "target" to which reference is made in this invention must be construed in its widest meaning; in particular, it can be a fluid flowing into a tube. This invention can have an excellent application in the medical field; the fluid in this case is the blood flowing in an artery or in a vein and the target is thus an elementary volume of this blood in a specific site of the human body, for example, close to the heart. The problems that are raised by measuring the speed of a moving target are set out and overcome herein-below in such an application of the medical type.
2. Description of the Prior Art
When the speed of a moving target is measuring by the Doppler effect by utilizing ultrasound techniques, a sonic or acoustical pulse of a relatively brief duration is emitted in the direction of this target. This pulse is propagated from an emitting probe through the environment that surrounds the target and is reflected onto this target in a sonic or acoustical pulse. It is retrodiffused in all directions and in particular in the direction of a remote sensing element or measuring pick-up. The sensors utilized are generally of a reversible type which means that a single sensor can emit the sonic or acoustical pulse and subsequently receive the reflected pulse. If the emitted pulse vibrates at a frequency f.sub.o the reflected pulse vibrates at a frequency that is shifted relatively to f.sub.o by the Doppler effect. The amplitude of this shift measures the radial component of the target speed along the outward-inward axis of propagation of these pulses. Since there is not only the target which reflects the pulse, it is known to open a receiving time window at the end of a determined duration following the beginning of the pulse emission, to receive the reflected sonic or acoustical vibration which corresponds to the target and only to the target. This determined duration corresponds to the ratio of the distance separating the target and the sensor at the speed of propagation of the pulses in the environment that surrounds the target.
Indeed, instead of emitting a single pulse and receiving a single pulse, sonic or acoustical pulses are periodically emitted and reflected pulses are periodically received. This emitting periodicity is dependant, on the one hand, upon the will of the user to know the speed of the blood flow in different places of a studied organ; in this case, the receiving window is displaced in time from one measurement to the other. On the other hand, since all the particles of an elementary volume of a fluid are not displaced at the same speed, it is thus necessary to calculate the mean speed of the elementary volume at one place involved and thus to carry out the spectral analysis of the reflected pulse which in the present state of the art cannot be carried out by receiving a single pulse but can only be carried out by combining the measurement of several successive reflected pulses.
The immediate consequence of this periodicity is to limit the admissible Doppler effect which can be directly measured. In fact, operating proceeds in the same way as if the phenomen studied, i.e. the blood in movement, was subjected to a stroboscopic illumination. The elementary volumes, the speed of which provokes a Doppler shift substantially equal to the recurrence frequency, are seen substantially as not moving. The elementary volumes which are displaced slightly more rapidly than a nominal speed for which the Doppler shift is equal to the half recurrence frequency, are even interpreted as being displaced in counter direction to their true displacement. The phenomenon is reproduced, of course, for every Doppler shift that is a whole multiple of the half recurrence frequency. If the user does not have access to the true speed, he only has access to the measured speed: this means that for this user there is always an ambiguity. This ambiguity raises the question as what is the true speed of the elementary volume studied while being aware of the measured value obtained.
Two types of solutions are known in the prior art that attempt to overcome this problem. The first solution consists in increasing the recurrence frequency F.sub.r so as to bring as high as possible the ambiguity threshold. If we call the measurement of the Doppler shift corresponding to a given speed f.sub.m, the nominal speed, i.e. that which can be admissibly measured, is such that f.sub.m =F.sub.r /2. Consequently, the more the recurrence frequency increases, the further the ambiguity threshold is pushed towards high values: if possible beyond the range of speeds that it is desired to measure. The first technique has two drawbacks. The first drawback is on the physiological level since the more the recurrence frequency increases, the higher the number of pulses sent into the patients body. It is questionable whether at high doses the ultrasonic excitations do not destroy certain cells of the human body. The second drawback is that the fact of increasing the recurrence frequency correlatively diminishes the period that separates two successive excitation pulses. This limits the time adjustment range of the position of the time receiving slot or window. In other words, it is the exploration depth from the sensor which is reduced.
In certain cases there can even be measuring errors linked to the fact that during the time receiving slot a signal is received that corresponds to the reception of an emitted pulse which is reflected at a given depth of the studied environment and which also corresponds to the reception of a preceding pulse but reflected in a deeper zone of this environment. In other words, if the exploration depth obstacle can be overcome, it remains that the measuring result takes simultaneously into account the interference of the deeper parts which falsifies said measurement. In a practical example, if frequency f.sub.o is equal to 7 MHz and if the radial component of the speed of the blood is about 3 m/s, in the human body, the speed of propagation is about 1500 m/s, the corresponding Doppler shift is about 30 KHz and the maximal exploration depth is about 2.5 cm. Thus, the radial speed of 3 m/s is representative of a blood flow speed of about 5 m/s if the axis of propagation of the pulses is oriented substantially at 60.degree. with respect to the means direction of the blood at the place of measurement (which corresponds to a general experimentation mode). Since the patients to be examined are frequently patients suffering from heart trouble, the true speed of the blood can be exceedingly higher (due to the fact, especially of aortic stenosis). Therefore, even in the case where the recurrence frequency is high and where the exploration depth is reduced, measurement ambiguities are possibly encountered.
A second technical solution consists in causing to drop the frequency of the sonic or acoustical signal emitted. If f.sub.o is this sonic or acoustical frequency, the Doppler shift f.sub.d can be expressed in the form of EQU f.sub.d =2.vf.sub.o /c
in which v is the speed to be measured and c is the speed of propagation of the sonic or acoustical pulse in the environment. For a given speed v, f.sub.d will be that much weaker as f.sub.o is itself low. The drawback of this choice is that there is a important loss of sensitivity on the determination of f.sub.d.
The object of the present invention is to overcome the drawbacks cited herein-above by proposing a process in which the signal representing the Doppler shift to be measured is modulated by a signal at a known frequency. The effect of this is to tranpose in the field of the frequencies the ambiguity thresholds and to allow the elaborate a measurement called a shifted measurement of the true value. The true value is found by correcting the shifted value of a value corresponding to the modulating frequency introduced.
The object of the invention is a process for removing all ambiguity from the measurement by Doppler effect of the speed of a target in which:
a pulse vibrating at a sonic or acoustical frequency is emitted in the direction of the target, periodically at a recurrence frequency; PA1 the ultrasonic pulse retrodiffused by the target is received; PA1 a complex demodulation of the signal received is carried out, by a signal of the same frequency as the emitting sonic or acoustical frequency; PA1 the speed of the target is computed by spectral analysis from the measurement of the Doppler shift signal resulting from this demodulation,--this shift being representative of the speed of the target--, the determination ambiguity of the speed of the target being induced by the importance of this shift with respect to the half recurrence frequency; PA1 the said demodulated signal is modulated by a signal oscillating at a frequency called determination frequency so as to transpose, from a known value, the Doppler shift to be measured in a band smaller than the half recurrence frequency; PA1 and in that the computing of the speed of the target consists in calculating by spectral analysis a speed called shifted speed from this modulated signal and in correcting the said shifted speed by a value corresponding to the determination frequency in order to elaborate the true value of the speed to be measured.